The present invention relates to extracorporeal systems for oxygenating and pumping blood during cardiac surgery. More specifically, the present invention relates to an integrated oxygenator and pump system wherein the gas diffusion fibers form a pumping element.
Each year hundreds of thousands of people are afflicted with vascular diseases, such as arteriosclerosis, that result in cardiac ischemia. For more than thirty years, such disease, especially of the coronary arteries, has been treated using open surgical procedures, such as coronary artery bypass grafting. During such bypass grafting procedures, a sternotomy is performed to gain access to the pericardial sac, the patient is put on cardiopulmonary bypass, and the heart is stopped using a cardioplegia solution.
Recently, the development of minimally invasive techniques for cardiac bypass grafting, for example, by Heartport, Inc., Redwood City, Calif., and CardioThoracic Systems, Inc., Menlo Park, Calif., have placed a premium on reducing the size of equipment employed in the sterile field. Whereas open surgical techniques typically provide a relatively large surgical site that the surgeon views directly, minimally invasive techniques require the placement of endoscopes, video monitors, and various positioning systems for the instruments. These devices crowd the sterile field and can limit the surgeon""s ability to maneuver.
At the same time, however, the need to reduce priming volume of the oxygenator and pump, and the desire to reduce blood contact with non-native surfaces has increased interest in locating the oxygenator and pump as near as possible to the patient.
In recognition of the foregoing issues, some previously known cardiopulmonary systems have attempted to miniaturize and integrate certain components of cardiopulmonary systems. U.S. Pat. Nos. 5,266,265 and 5,270,005, both to Raible, describe an extracorporeal blood oxygenation system having an integrated blood reservoir, an oxygenator formed from a static array of hollow fibers, a heat exchanger, a pump and a pump motor that is controlled by cable connected to a control console.
The systems described in the foregoing patents employ relatively short flow paths that may lead to inadequate gas exchange, due to the development of laminar flow zones adjacent to the hollow fibers. U.S. Pat. No. 5,411,706 to Hubbard et al. describes one solution providing a longer flow path by recirculating blood through the fiber bundle at a higher flow rate than the rate at which blood is delivered to the patient. U.S. Pat. No. 3,674,440 to Kitrilakis and U.S. Pat. No. 3,841,837 to Kitrilakis et al. describe oxygenators wherein the gas transfer surfaces form an active element that stirs the blood to prevent the buildup of boundary layers around the gas transfer surfaces.
Makarewicz et al., xe2x80x9cA Pumping Intravascular Artificial Lung with Active Mixing,xe2x80x9d ASAIO Journal, 39(3):M466-M469 (1993), describes an intravascular device having a gas exchange surface made of microporous fibers formed into an elongated helical vane. The elongated helical vane permits not only gas exchange, but also may be rotated to pump blood through the device.
Makarewicz et al., xe2x80x9cA Pumping Artificial Lung,xe2x80x9d ASAIO Journal, 40(3):M518-M521 (1994) describes an adaptation of the foregoing device in which the microporous fiber bundles were formed into multi-lobed clover-leaf vanes potted along a central axis. The vanes were substituted for the vanes of a BIOMEDICUS(copyright) blood pump (a registered trademark of Bio-Medicus, Eden Prairie, Minn.). The authors concluded that while the concept of achieving simultaneous pumping and oxygenation appeared feasible, additional design and testing would be required, and problems, such as hemolysis and platelet activation, must be addressed.
Makarewicz et al., xe2x80x9cNew Design for a Pumping Artificial Lung,xe2x80x9d ASAIO Journal, 42(5):M615-M619 (1996), describes an integrated pump/oxygenator in which a hollow fiber bundle replaces the multi-lobed vanes of the above-described design. The hollow fiber bundle is potted to an inlet gas manifold at the bottom, and an outlet gas manifold at the top. The fiber bundle is rotated at high speed to provide pumping action, while oxygen flowing through the fiber bundle oxygenates the blood.
U.S. Pat. No. 5,830,370 to Maloney et al. describes a device having a fiber bundle mounted for rotation between a fixed central diffuser element and an outer wall of a housing. The fiber bundle is rotated at speeds sufficiently high to cause shear forces that induce turbulent flow within the blood. The patent does not address or even recognize the problem of blood trauma, i.e., hemolysis and platelet activation, that is expected to result from turbulent, high shear flow.
Although the devices described in the foregoing references offer some desirable features, those devices have numerous drawbacks that make them commercially impractical. These problems include: (a) introduction of small bubbles (xe2x80x9cmicrobubblesxe2x80x9d) into the blood from the fiber due to higher gas side pressure relative to blood side pressure; (b) cavitation-induced blood trauma and damage to the device; (c) high shear loading leading to (i) buckling of the fibers or (ii) blood trauma; and (d) flooding of the inlet gas manifold, after fiber rupture, resulting in rapid reduction in oxygenation efficiency.
In view of the foregoing, it would be desirable to provide an extracorporeal blood pump/oxygenator that provides compact size, low priming volume, low surface area, and the ability to adequately oxygenate blood using a rotating fiber bundle that reduces boundary layer transfer resistance.
It would be desirable to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle that oxygenates the blood and provides pumping action when rotated, but does not suffer from the leakage of gas into the blood, which leads to undesirable bubble formation.
It also would be desirable to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle that oxygenates the blood and provides pumping action when rotated, but which overcomes microbubble generation problems observed in previously known integrated pump/oxygenator systems.
It further would be desirable to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle design that does not generate high shear stresses, and thus is less susceptible to shear stress-induced fiber breakage and consequent leakage.
It would be further desirable to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle design that reduces shear-induced blood trauma, including hemolysis and platelet activation.
It still further would be desirable to provide an integrated extracorporeal blood pump/oxygenator having a rotating hollow fiber bundle that is less susceptible to flooding of the gas manifolds than previously known designs.
It further would be desirable to provide an integrated extracorporeal blood pump/oxygenator having a low priming volume, thus making the system suitable for emergency back-up operation.
In view of the foregoing, it is an object of the present invention to provide an integrated extracorporeal blood pump/oxygenator having a compact size, low priming volume and the ability to adequately oxygenate blood using a rotating fiber bundle that reduces boundary layer resistance to gas transfer and the formation of stagnation zones within the fiber bundle.
It is another object of the present invention to provide an integrated extracorporeal blood pump/oxygenator having a low priming volume and low internal surface area, thereby reducing blood contact with non-native surfaces, potential damage to blood components, and the risk of infection.
It is yet another object of this invention to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle that oxygenates the blood and provides a pumping action when rotated, but reduces the leakage of gas into the blood to form bubbles.
It is a further object of this invention to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle that oxygenates the blood and provides a pumping action when rotated, but which overcomes microbubble generation problems observed in previously known integrated pump/oxygenator systems.
It is a still further object of the present invention to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle that is less susceptible to breaking or buckling of the fibers due to high shear forces on individual elements of the fiber bundle and consequent leakage.
It is yet another object of the present invention to provide an integrated extracorporeal blood pump/oxygenator having a hollow fiber bundle design that reduces shear-induced blood trauma.
It is yet another object of this invention to provide an integrated extracorporeal blood pump/oxygenator having a rotating hollow fiber bundle that is less susceptible to flooding of the gas manifolds than previously known designs.
It is another object of the invention to provide an integrated extracorporeal blood pump/oxygenator having a low priming volume, thereby making the system suitable for emergency back-up operation.
These and other objects of the invention are accomplished by providing an integrated blood pump/oxygenator, suitable for use within a sterile field, that has a low priming volume. In accordance with the principles of the present invention, the pump/oxygenator includes a rotating hollow fiber bundle assembly that both oxygenates the blood and develops additional pressure head, if desired, to pump the blood. The device further includes one or more of the following improvements: means for reducing microbubble generation and blood trauma; means for reducing outward bowing of the fiber bundle; and means for reducing flooding of gas manifolds.
In a preferred embodiment, the integrated blood pump/oxygenator includes an tapered inner member disposed along a central shaft that increases pressure on the blood side relative to the gas side near the center of the fiber bundle, and, hence, prevents the formation of gas microbubbles in the blood. The inner member, which may optionally include helical vanes, also gradually accelerates blood prior to entering the fiber bundle, thereby reducing blood trauma. Shearing loads imposed on the fiber elements of the fiber bundle during high speed rotation are addressed by the addition of a reinforcement structure that extends around or within the fiber bundle. These reinforcement structures also assist in reducing shear stress imparted to the blood, hence reduce blood trauma. In addition, the gas manifolds of the pump/oxygenator optionally may be configured to reduce flooding and loss of efficiency due to occasional rupture of fiber elements.
Alternative embodiments of the integrated blood pump/oxygenator of the present invention may include a plurality of vanes for accelerating blood entering and/or exiting the fiber bundle. These vanes may be coupled to the same shaft that drives the rotating fiber bundle, or may optionally be driven at a different angular velocity using a separate drive train.